Inter-carrier noise suppressor circuit



AIIEQ '6, 1935- 2 L. E.BQARTON 2,010,252

INTERCARRIER NOISE SUPRRESSOR CIRCUIT Filed NOV. 22, 1935 pmrfmus 1mm 1L7??- 1w Z Q .1 55 Q 7'2 P a 1AA. 14.54.

5 7 1+5 AJZC.

INVENTOR LOY' Ev BARTON ATTORNUEY Patented Aug. 6, 1935 INTER-CARRIER. TNOISE .aSUPPRESSOBI'" CIRCUIT- Loy E. Barton, Collillgswood, N. J., assignor'to Radio CorporatiomofAmerica; a corporation" of Delaware I ApplicationNovember-ZZ, 1933,. Serial No:699,1-77

, 6 claims-w (o1. zsomzo) My. present invention relates to gain control circuits. for radio. receivers, and more .particularly to a novel type ofinterstation noise suppressor" network for a radio receiver I Background noise suppressor arrangements, ormutingnetworks, are wellknown at therpresent time, and are espeoiallyutilizedinradio receivers which .employ. automatic volume controls. It is tion,"but1-; economically,-manufactured -and as-w well known that the function of the muting de- Vice is to renderthe receiver inefficient to reproduce undesired background noise impulses when the receiver .is tuned between a pair ofadjacent station frequencies. The suppressor action is employed between adjacent desired,sta.- tion settings because in thistintermediate portionof the tuning spectrum undesired background noiseiznpulses arereproduced toan annoying ex-. tent byvirtue of the maximum sensitivity of the high .frequency amplifiersauxiliary high frequency channels to .feedthe suppressor network, .and .haveadditionally. utilizedauxiliary electronic devices for performing the. muting function...,Again, the silencing has-been accomplished in an audio frequency portionof the Accordingly, it may .berstated that it is 1 receiver. one of the main objects of the presentinvention toiprovide a highly efiicient silencing device in a radio receiver embodying automatic lvolume, control, which silencing device permits noise suppression without the usetof additional high 'frequency channels or auxiliary electron discharge the demodulator anelectrondischargetube, normally functioningas a radio frequency amplifier, which possesses a low amplification factor at low plate voltages, the amplification factor rapidly rising with increased plate. voltages, the last named amplification. factor characteristic of the tube being taken, advantage of in performing interstation noise suppression...

Another object oftheipresentl invention is .'to.-

provide a radioufrequency amplifier stage between thesignal collectorof a receiver andithe signal .-demodulator,..the .amplifierbeing of the .pentodei typeand employing a resistorwiniits-ianode circuit, whichw resistor. has amagnitude so proportionedin respect tolthe plate currenteplatevolt-L'. agecharacteristics o f the pentodetube thatnore, a mal. s transmission of; signals: through saidi Commercial broadcast receivers have employed I although. those skilled in the art will underirequencyasignalsw The receiver-may employ acquainted with the circuit design. The first. detector-and i local oscillator network= may,com-,.

. pentode amplifier is substantially prevented.- -be-';

tween; adjacent -statiorri settings 'of the' tunings;-- means of'the receiven.

Still .other objects of -the invention are to ima prove generally-the simplicity and-efficiency of intercarrier noise suppressorarrangements for r radio receiversyand especiall'ytoprovidesuch an arrangement which is not only: reliable in opera.

sembled in a radio receiver. 1

The novel -features1- which. I- believe to bevchars acteristic of myrinvention are set-r forthiincpara ticularity in the appended claims; The -inven-.-=:-' tion itself; however; 'both as vto its organizaw tion and methodof operation: will best-be under-.- 1

stood by reference "to Hthe ;following-.description taken in connection with the drawing in which I .have indicated 1 diagrammatically a circuit i organization whereby myw invention may be canriedinto effect:-

In' the. drawing,

Figi l graphically shows-characteristics of-a tube:- employing --the invention,

Fig. 2. diagrammatically shows a receiverwhich embodies the :present invention.-

Thetinterstation background noise suppressor I arrangement disclosed in this application .is em- .bodied in. a radio :receiver; preferably 0f the broadcast. type, insa mannerdiagrammaticallyillustrated inFig; 2. The receiver-systemshown in the latter is of a 'purely conventional type, t

and functions according .to the superheterodyne-r principle of .receptiorrm The signal selector may comprise -the usualcgrounded antenna circuit i stand that; the signal collector 1 4 may consistv .-of-. a loop .antenna or even a line -source ofradio at -.leastone stage-of tunable radio frequency amplification; a succeeding: first detectors andlocal oscillator. followed by--anintermediate :frequency amplifier the amplified intermediate-ha quency being. demodulated in a secondrdetector, stage, and the demodulated energyqbeing time? pressed upon an, audio. frequency. amplifier and/or-reproducer.

The oircu'itdetails of :the networks. follow-- ing the radio frequency.amplifier.tub.e..2 arenot shown because -those skilledin the artsare well prise separatetubesfor the .two.functions,,or..a .l. combined. first detector-local oscillator stage, may be utilized. In. anycase, it is .to be clearly'.

iunderstood. that the referencenumeraL-3 desig -fllifipy hates the tuning instrumentality of the first detector and local oscillator, and that this tuning instrument is uni-controlledly operated with the tuning means t of the radio frequency amplifier in any manner well known to those skilled in the art, the uni-control means being shown by the dotted line. The second detector network preferably embodies a duplex diode triode tube, that is to say a tube of the 55 type, and one of the diodes of the tube is used for automatic volume control of the radio frequency amplifier tube 2. The automatic volume control path is designated AVC, and the resistor 5 func tions as a ripple filter resistor.

It is preferred to employ an arrangement of the type disclosed and claimed in my co-pending application Serial No. 640,946, filed November 3, 1932, since this co-pending application discloses a second detector network of the type desired to be used. It is sufficient for the purposes of this application to point out that one of the diode sections of the multiple function tube 55 functions as a signal demodulator, while the other diode section operates to accomplish the automatic volume control function when its anode assumes at least cathode potential or a more positive potential. The triode portion of the tube 55 functions as an audio frequency amplifier, and derives the audio component from the demodulated diode network.

Regardless of the construction of the second detector network and the automatic volume control arrangement, the fact of importance in this application is that when the incoming signal carrier amplitude drops too low, the radio frequency amplifier tube 2 has its control grid biased at a minimum negative potential. This means that the amplifier is operating at its maximum sensitivity, and therefore will amplify undesired background noise impulses to a relatively greater extent. Gf course, as the carrier amplitude increases and approaches a maximum, the automatic volume control arrangement acts to increase the negative bias on the control grid of tube 2, with the result that the sensitivity, or amplification, of the radio frequency amplifier is reduced.

Now, if the signals impressed upon tube 2 suddenly disappear, as by excessive fading or when tuning between two adjacent station settings, the amplifier 2 is operating at maximum sensitivity and the only energy impressed upon the tube is the undesired background noise impulses. Hence, these impulses will be reproduced, and prove to be exceedingly annoying to the user of the receiver. It is this situation that background noise suppressor arrangements have been employed to eliminate. Heretofore it has been done by means of auxiliary signal channel and tubes. In the present arrangement the muting action, that is the silencing of the reproduction of the background noises, is accomplished by merely inserting a resistor R. of a proper value in the anode circuit of tube 2.

The anode of tube 2 is connected to the proper positivepotential point on the source B, which source is not shown, through a path which includes the primary 6 of the coupling transformer feeding into the first detector, the resistor R and the adjustable tap l. The tube 2 is of the 58, or 39, type. That is, it is a radio frequency pentode tube and has a suppressor grid disposed between the positive screen grid and the anode, the suppressor grid being connected within, or without, the tube envelope to the cathode. This type of tube is well known to those skilled in the art, and, in general, may be characterized as an electron discharge tube with a low amplification factor at low plate voltage, the factor rapidly rising with increased plate voltages. The characteristics of the 39 and 53 type tubes, or tubes of similar characteristics, are such that if a plate resistor of a relatively high value is inserted in the plate circuit with low control grid bias, the resulting plate voltage is very low and limits the gain of the tube to a very low value. A slight gain is still obtained at this point so that sufficient signal may get to the automatic volume control system to supply additional bias to the pentode tube. The additional bias decreases the plate current, with the result that the gain increases. This action is accumulative until the automatic volume control system reduces the gain by increases in bias above the point of maximum gain in the pentode suppressor tube.

The characteristic curves shown in Fig. 1 will now be explained in order to demonstate clearly the operation and theoretical basis of the present invention. In this figure plate current values, in milliamperes, are plotted as ordinates against plate voltages as abscissae. The different curves obtained correspond to different values of pentode control grid bias. Thus, the uppermost curve results from a negative grid bias Eg of zero volts. The lowermost curve results when the grid bias is -30 volts. The variation of tap 7 along resistor R regulates the degree of noise suppression. The control for the degree of suppression desired may be a rheostat of zero to 100,000 ohms in the plate circuit, or a fixed resistance of about 40 to 100,000 ohms in the plate circuit with a screen grid voltage control. That is to say, if a fixed resistor R is employed, then the screengrid voltage should be adjustable to vary the effective plate impedance of the tube.

Returning now to the characteristic curves of Fig. 1, let it be assumed that the resistor R has a value R2 of about 50,000 ohms, and that the line R2 in Fig. 1 represents the function of this resistor value. It will be seen that at l to zero bias on the grid of pentode tube 2 the gain through this radio frequency amplifier tube is extremely low, butsound signals get through. If the input signal is sufficiently large it will reach the second detector, and cause an automatic volume control voltage to be applied to the grid of tube 2. This injturn causes the platevoltage of tube 2 to in crease along line R2. Initial increases in bias in this tube increases its gain which will further increases its bias and so on until the maximum gain is reached. After the bias is increased to a point B on line R2, approximately maximum gain is obtained and further increases in bias reduce the gain of the tube 2 in the usual manner.

Thus, it will be seen that a signal sufiiciently large will trigger the system off. That is, a signal sufiiciently large to cause automatic control voltage, will bring the receiver quickly to a point of normal operation. The signal required to release the suppressor depends upon the value of the resistance in series with the plate of tube 2. That is, a much larger signal will be required for the resistance value R2 than is required, for example, for the resistance value R1, which means that the value R2 will suppress higher amplitude noises than the value R1. The most satisfactory operation will be had when the resistor R is of such value that the noise is reduced to the desired point. That is the value of resistor R should be as small as possible in order that the maximum number of stations may be heard without moving the tap 1.

It will be noted that after the suppressor is released a much lower signal may be received before suppression is obtained than is required for release. Thus, if a signal is just strong enough to release the suppressor, the signal may fade badly without the suppression taking effect. The present noise suppressor arrangement accomplishes the muting action at the radio frequency portion of the receiver without the utilization of additional channels or tubes. When all of the resistance of resistor R is used, plate voltage to the tube is very low whereby little plate swing can be obtained until the second detector supplies a bias which permits higher plate voltage.

It is to be clearly understood that the present invention is not limited in any way to the securing of the noise suppressor function in the tube immediately following the signal collector, since a pentode tube could be employed in the intermediate frequency amplifier and a similar function secured. Furthermore, it is entirely possible to utilize the present invention in the audio frequency section of the receiver. While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:-

1. In a radio receiver, a signal demodulator, a signal amplifier coupled between a signal collector and said demodulator, said amplifierineluding a tube having an amplification factor which is low at low plate voltages and which rapidly rises with increased plate voltages, a properly chosen impedance in the anode circuit of said signal amplifier tube, the magnitude of said impedance being such that the gain of said signal amplifier tube automatically decreases to prevent normal transmission therethrough when signals below a predetermined carrier amplitude are impressed on said amplifier tube and the amplifier bias is varied in a positive direction to a predetermined value, and means responsive to signal amplitude changes at the demodulator input for adjusting the said amplifier bias to said last named value.

2. In a radio receiver, a signal demodulator, a signal amplifier coupled between a signal collector and said demodulator, said amplifier including a tube having an amplification factor which is low at low plate voltages and which rapidly rises with increased plate voltages, a resistive impedance in the anode circuit of said signal amplifier tube, the magnitude of said impedance being such that the gain of said signal amplifier tube automatically decreases to prevent normal transmission therethrough when the amplifier bias is varied in a positive direction to a predetermined value, and means for adjusting the bias to said value.

3. In a radio receiver, a signal demodulator, a signal amplifier coupled to said demodulator, said amplifier including a tube having an amplification factor which is low at low plate voltages and which rapidly rises with increased plate voltages, a properly chosen impedance which consists of a variable resistor in the anode circuit of said signal amplifier tube, the magnitude of said impedance being such that the gain of said signal amplifier tube automatically decreases to prevent normal transmission therethrough when the amplifier bias is varied in a positive direction to a predetermined value, and means for adjusting. the bias to said value.

4. In a radio receiver, a signal demodulator, a signal amplifier coupled to said demodulator, said amplifier including a pentode tube having an amplification factor which is low at low plate voltages and which rapidly rises with increased plate voltages, a properly chosen impedance in the anode circuit of said signal amplifier tube, the magnitude of said impedance being suchthat the gain of said signal amplifier tube automatically decreases to prevent normal transmission therethrough when signals below a predetermined carrier amplitude are impressed on said amplifier tube and the amplifier bias is varied in a positive direction to a predetermined value, and means for adjusting the bias to said value.

5. In a radio receiver, a signal collector, a radio frequency amplifier including a tube having an amplification factor which is low at low plate voltages and which rapidly rises with increased plate voltages, at tuned circuit connected between the input electrodes of said tube, a detector coupled to the anode circuit of said tube, means for automatically regulating the gain of said amplifier tube in response to variations in received signal carrier amplitude in such a manner as to maintain the signal input to the said detector substantially constant, and a resistor in the anode circuit of said radio frequency ampli-' fier which has a magnitude such that the gain of said radio frequency amplifier is automatically made a minimum when no signals are impressed upon the receiver.

6. In a radio receiver, a radio frequency amplifier of the pentode type, a variable resistor in the anode circuit of the pentode tube, a signal demodulator coupled to the anode circuit of said pentode tube, said resistor being adjustable to a value such that the gain of the amplifier varies. in the same sense as the amplifier negative bias, and means for automatically regulating the said bias of said pentode tube in response to variations in received signal carrier amplitude, said amplifier gain increasing with said bias only up to a point of maximum efliciency and then decreasing.

LOY E. BARTON. 

